1. Field of the Invention
The present invention relates to a clipping processor for clipping a polygon unlocated on a screen plane in an image processor for synthesizing and outputting an image signal outputted from an image information supplying source in real time.
2. Description of the Related Art
In an image processor of three-dimensional graphics, polyhedral data are represented as an aggregate of point information in which vertex coordinates of a polyhedron are respectively changed by a transparent projecting transformation to two-dimensional coordinates of X and Y. In such a transparent projecting transformation, a distance between a starting point and each of the vertex coordinates of the polyhedron is calculated in advance. In general clipping processing in the three-dimensional graphics, when an intersecting point of a polygon and a screen is calculated, it is necessary to calculate an intersecting point at a Y-coordinate on a screen boundary in accordance with a complicated formula.
Therefore, it takes much time to perform a clipping operation. Otherwise, large-sized hardware is required.
In a general clipping processor, an acceptable coordinate range is set to be larger than a screen region. Accordingly, a memory having a large capacity is required and a memory access time is increased.
In another clipping processor in which clipping processing can be performed by using small hardware at a high speed, when X and Y addresses on a polygonal side are calculated, all the X-addresses are set to an X-address at a starting point on the screen when the X-addresses are smaller than the X-address at the starting point on the screen. In contrast to this, all the X-addresses are set to an X-address at a terminal point on the screen when all the X-addresses are greater than the X-address at the terminal point on the screen. Further, when the Y-address on the polygonal side is smaller than a Y-address at the starting point on the screen, or is greater than a Y-address at the terminal point on the screen, a data-writing operation is stopped at each of the starting and terminal points shown by the X-address at this Y-address. Thus, it is possible to reduce a hardware amount and perform the clipping processing at a high speed.
The clipping processing and the calculation of X and Y addresses of a polygon are simultaneously executed so that concurrent processing can be performed when the clipping processor is constructed by hardware, thereby operating the clipping processor at a high speed. Further, it is possible to construct the clipping processor by a small hardware amount without requiring any divider, etc. when the clipping processor is constructed by hardware.
A general clipping method can be applied in the case of a polygon in which a polygonal face has a single color. However, no general clipping method can be applied in the case of a mapped polygon in which a pattern is formed on the polygonal face.
Further, in the general clipping method, with respect to a polygon unlocated on a screen face, it is necessary to make an address calculation about each of dots on all polygonal sides, thereby performing many useless processings.
When Z-sorting processing is performed after the clipping processing, an unprocessed polygon is caused by an upper limit of number of polygons processed in the previous Z-sorting processing.